Introducing cyclic alkyl chains into small-molecule acceptors for efficient polymer solar cells

Article abstract of DOI:10.1039/c8tc01780h

A new acceptor-donor-acceptor type small molecule acceptor, namely IDT-HN, has been developed, which consists of a newly developed 2-(3-oxo-2,3,5,6,7,8-hexahydro-1H-cyclopenta[b]naphthalen-1-ylidene)malononitrile as the peripheral electron-withdrawing gro

Full text of DOI:10.1039/c8tc01780h

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10.1039/C8TC01780H.
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Page 1 of 28
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DOI: 10.1039/C8TC01780H
Introducing Cyclic Alkyl Chain into Small-Molecule
Acceptor for Efficient Polymer Solar Cells
Renlong Li,‡ Gongchu Liu,‡ Ruihao Xie, Zhenfeng Wang, Xiye Yang, Kang An,
Wenkai Zhong, Xiao-Fang Jiang, Lei Ying,* Fei Huang,* and Yong Cao
State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer
Optoelectronic Materials and Devices, South China University of Technology,
Guangzhou 510640, P. R. China.
E-mail: msleiying@scut.edu.cn; msfhuang@scut. edu.cn.
†Electronic Supplementary Information (ESI) available: Details of experimental
section, device fabrication and characterization, carrier mobility measurement,
other figures and tables. See DOI: 10.1039/x0xx00000x.
‡These authors contributed equally to this work.
1

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Abstract: A new acceptor-donor-acceptor type small molecular aDcOceI: p10t.1o0r39,/C8TC01780H
namely IDT-HN, is developed, which consists of a newly developed
2-(3-oxo-2,3,5,6,7,8-hexahydro-1H-cyclopenta[b]naphthalen-1-ylidene)malonon
itrile
as
the
peripheral
electron-withdrawing
group
and
indaceno[1,2-b:5,6-b′]dithiophene as the electron-donating core. Compared
with the reference molecule (IDT-IC) that bears phenyl-fused indanone as the
end groups, IDT-HN exhibited elevated lowest unoccupied molecular orbital
level. By utilizing IDT-HN as the electron acceptor and a wide bandgap
conjugated polymer PBDB-T as the electron donor, optimized devices exhibited
an impressively high power conversion efficiency of up to 10.22% with
simultaneously improved open-circuit voltage, short-circuit current and fill
factor. The improved photovoltaic performance can be attributed to the
widened and intensified absorption spectra, improved electron mobility, more
ordered π-π packing structure, and the symmetric carrier transport mobility of
IDT-HN-based blend with regard to those obtained from devices based on the
reference molecule IDT-IC. These results indicate that the incorporated cyclic
alkyl moiety into the peripheral groups plays a critical role in improving the
performance of the corresponding solar cell devices. In addition, the power
conversion efficiency of the devices remains at 91% of its optimum performance
with film thickness of 250 nm, indicating a great potential for future practical
application.
Keywords: Non-fullerene organic solar cells, small molecular acceptor, novel
electron-withdrawing end group, cyclic alkyl chains, high fill factor
2

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DOI: 10.1039/C8TC01780H
1. Introduction
In the past two decades, solution-processed organic solar cells (OSCs) with bulk
heterojunction (BHJ) structure have attracted much academic and industrial
interest, primarily due to their extensive advantages such as mechanical
flexibility, low-cost production, chemical versatility, and solution processability
for fabrication of large-area devices.1-5 For the electron-accepting material
component, the recently emerged non-fullerene acceptors (NFAs) have shown
the especially promising characteristics of better absorption, easily tunable
6 - 33
energy levels, and facile accessibility.
The specific merits of such
small-molecule NFAs include well-defined molecular structures and molecular
weights, and they can be prepared in high purity that can avoid batch-to-batch
variations. Recently, benefiting from the development of NFAs, the power
conversion efficiency (PCE) of OSCs has been improved to over 13%.17,27,34
Small-molecule acceptors (SMAs) with acceptor-donor-acceptor (A-D-A)
type
molecular
structure
(such
as
3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-
hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene, ITIC,
and its derivatives) have been extensively used in OSCs to achieve high
performance.23,34, 35 To maximize the PCE, much work has focused on the
modification
of
the
powerful
electron-withdrawing
end
group
2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (INCN) via chemical
methods to improve the short-circuit current (JSC) 23,36, open-circuit voltage
3

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DOI: 10.1039/C8TC01780H
(VOC)26, and fill factor (FF)35, as PCE is the product of these parameters.
However, it remains great challenge to simultaneously improve all of these
parameters, because in many cases the molecular properties and film
morphology have opposite impacts on these factors. For instance, to achieve
higher JSC, various electron-deficient units (such as fluorine atoms) have been
introduced into the phenyl moiety of INCN to enhance the intramolecular charge
transfer effect and broaden the absorption spectrum of the SMAs. Although JSC
can be greatly improved when the modified SMAs are incorporated into OSCs, the
corresponding VOC decreased due to the decreased lowest unoccupied molecular
orbital (LUMO) energy level of the acceptor.23 Alternatively, to achieve higher VOC,
electron-donating groups (such as methyl and methoxy) have been introduced
into the phenyl moiety of INCN to elevate the LUMO of the SMAs; however, the
corresponding absorption spectra of the SMAs are blue-shifted, which is not
favorable for the conversion of solar photons to electrons.26 Regarding to the
aforementioned two parameters, the achievement of high FF is of particular
challenging, as it depends on multiple factors including the morphology of the
BHJ layer, balance of charge carrier mobility, recombination of charge carrier, and
so forth.37,38,39
Very
recently,
we
reported
an
A-D-A-type
acceptor
2,2′-((2Z,2′Z)-((4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6
-b′]dithiophene-2,7-diyl)bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-cyclope
nta[b]naphthalene-2,1-diylidene))dimalononitrile (namely IDT-N, Figure 1a)
4

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consisting of indacenodithiophene (IDT) as the electron-rich coDrOeI: 10a.1n03d9/C8TC01780H
40
naphthyl-fused indanone (N) as the electron-withdrawing end groups.
Comparing
IDT-N
with
the
previously
reported
IDT-IC
(2,2'-((2Z,2'Z)-((4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,
6-b']dithiophene-2,7-diyl)bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-inden
e-2,1-diylidene))dimalononitrile), which has phenyl-fused indanone (INCN) as
the end groups, the performance of the device based on PBDB-T:IDT-N (8.99%)
was higher than for the device based on PBDB-T:IDT-IC (6.11%), mainly due to
the improved JSC and FF. However, the VOC was decreased from 0.88 V to 0.79 V,
which limited the photovoltaic performances. Recently, Yang et al. demonstrated
that cyclic chains adopt preferential conformations in the crystals of
π-conjugated polymers, which can promote the molecular self-assembly and
packing properties, 41 and thus leading to more ordered micro-structures
enhanced the absorption of light by the film and the balance of electron/hole
transport that can simultaneously improve JSC and FF. In this respect, it seems
rational that the LUMO levels of SMAs can be enhanced upon introducing cyclic
alkyl chains into benzene unit of the electron-withdrawing INCN moiety that can
lead to a higher VOC, and the JSC and FF would be simultaneously improved.
In this work, we introduced a cyclic alkyl chain into the benzene unit of the
end group INCN to synthesize
a
new end group moiety, HN
(2-3-oxo-2,3,5,6,7,8-hexahydro-1H-cyclopenta[b]naphthalen-1-ylidene)malononi
trile. Using HN as the electron-withdrawing end group, a new small-molecule
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DOI: 10.1039/C8TC01780H
IDT-HN,
2,2′-((2Z,2′Z)-((4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6
-b′]dithiophene-2,7-diyl)bis(methanylylidene))bis(3-oxo-2,3,5,6,7,8-hexahydro-1
H-cyclopenta[b]naphthalene-2,1-diylidene))dimalononitrile (Figure 1a), was
synthesized. The LUMO energy level was up-shifted compared with the reference
molecule IDT-IC when using INCN as the end group, which can be attributed to
the electron-donating nature of the cyclic alkyl substituent that is favorable for
VOC. Importantly, IDT-HN showed a broadened and intensified optical absorption
regarding to the counterpart molecule IDT-IC due to the more ordered molecular
chain arising from stronger intermolecular interactions among the end groups,
which is favorable for JSC. Furthermore, IDT-HN exhibited a higher electron
mobility (µe) and a more balanced electron/hole transport in the blend film than
those of IDT-IC, due to the enhanced intermolecular interactions and the
optimized film morphology, which was beneficial for suppressing charge
recombination to improve the FF. Finally, the devices based on IDT-HN and the
polymer PBDB-T (Figure 1a) demonstrated a significantly improved PCE of
10.22%, with simultaneously improved VOC, JSC, and FF, compared with the
corresponding OSCs with IDT-IC as the acceptor. It is noteworthy that the
improved FF value of 76.41% is distinctly higher than that of most previously
reported high-performance NF-OSCs. Thus, these results endorse the
introduction of cyclic alkyl chains into SMAs as a promising strategy to design
higher-performance NF acceptors.
6

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(a)
Improve JSC and FF
But decrease VOC
Improve JSC, FF,
and VOC
Previouswork
This work
IDT-IC
1.6x105 (b)
1.2x105
80000
(c)
1
0.8
0.6
0.4
0.2
0
(d)
IDT-IC
IDT-HN
IDT-N
PBDB-T
IDT-HN
-3.53
-5.33
-3.86
-3.95
-5.84
-4.01
-5.86
40000
-5.92
0
300 400 500 600 700 800
Wavelength (nm)
300 400 500 600 700 800 900
Wavelength (nm)
Figure 1. (a) The molecular structures, and (b) absorption spectra of the acceptors in
chloroform solution, and (c) absorption spectra of the acceptors and the donor as thin films,
and (d) energy level diagram.
2. Results and Discussion
2.1 Synthesis and Characterization
Details of the synthesis of the molecule IDT-HN and the end group HN are
provided in the Supporting Information (Scheme S1, ESI†). The novel
electron-withdrawing moiety HN was synthesized via several steps using
low-cost N,N-dimethylformamide (DMF) and cyclohexanone as the starting
materials. The targeted compound IDT-HN was obtained by Knoevenagel
condensation reaction of the intermediate compound (IDT-CHO) and the
electron-withdrawing moiety (HN) in 90% yield. The molecular structure of
1
IDT-HN was characterized by H and 13C nuclear magnetic resonance (NMR)
spectroscopy and matrix-assisted laser desorption/ionization time of flight mass
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spectrometry (MALDI-TOF MS). IDT-HN proved to be very soluble in common
organic solvents, such as chloroform, chlorobenzene, and o-dichlorobenzene.
Thermogravimetric analysis revealed the good thermal stability of IDT-HN, with
the decomposition temperature of 363 °C, denoted as 5% weight loss (Figure S1a,
ESI†). Differential scanning calorimetry analysis revealed no obvious thermal
transition characteristics in the temperature range from 30 to 280 °C for IDT-HN
under nitrogen (Figure S1b, ESI†).
2.2. Theoretical Calculations
In order to explore the possible influence of the terminal groups of cyclic alkyl
chain (HN) on the geometry properties of the A-D-A-type small molecules,
density functional theory (DFT) calculations by using Gaussian 09 software at the
ωB97X-D/6-31G* level were carried out to evaluate the molecular geometry of
the ground state of the two acceptors.42 To facilitate the calculations, the long
alkyl chains of the two acceptors were replaced with methyl groups, respectively.
Figure 2a and 2b depict the geometry-optimized structures of the two acceptors
(IDT-IC and IDT-HN) as observed from the top and side views, both acceptors
were predicted to show a coplanar configuration in the conjugated backbone
moiety. Furthermore, on the basis of the optimized ground state structures,
natural transition orbitals (NTOs) were performed by using time-dependent (TD)
DFT functional at the ωB97X-D/6-31G* level. Figure 2c depicts both the occupied
and unoccupied NTOs with the largest contribution to the S0-S1 transition for the
8

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molecule IDT-HN, and π-electron delocalization occurs over the entire backbone.
On the other hand, the corresponding molecule IDT-IC also exhibits a coplanar
backbone configuration with NOTs well-distributed over the backbone under the
same calculation conditions (Figure S2, ESI†).
Figure 2. (a, b) Geometry-optimized structures of IDT-IC and IDT-HN; (c) representations of
the TD-DFT tuned-ωB97X-D natural transition orbitals with the largest eigenvalues of S0–S1
transition for IDT-HN (top: electron wave-function; bottom: hole wave-function).
2.3. Electrochemical and Optical Properties
As shown in Figure 1b, in chloroform solution, both IDT-IC and IDT-HN show
strong and broad optical absorption in the region of 500-700 nm. The maximum
extinction coefficient of IDT-HN (1.72 × 105 M−1 cm−1) is higher than that of
IDT-IC (1.40 × 105 M−1 cm−1). In thin films (Figure 1c and Figure S3b, ESI†), both
acceptors show red shifted and broadened absorption profiles compared with
the corresponding solution spectra, which can be attributed to the more
pronounced π-π intermolecular interactions in film state. The absorption peaks
of the IDT-HN and IDT-IC films are 671 and 680 nm with the corresponding
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extinction coefficients of 1.28 × 105 and 1.07 × 105 cm−1, respDeOcIt:i1v0.e10l3y9./C8TC01780H
Compared with the main optical absorption of IDT-IC (from 500 to 740 nm),
IDT-HN showed a wider and more intense absorption (from 480 to 740 nm) with
the absorption edge at the same position (around 740 nm), and much stronger
π-π intermolecular interactions. The broadened absorption and enhanced
extinction coefficient of IDT-HN are favorable for harvesting solar photons and
thus would result in improved JSC. Both acceptors have the same optical
bandgap of 1.68 eV, as calculated from the absorption onsets of the thin films.
Cyclic voltammetry (CV) measurements were performed to probe the frontier
molecular orbital energy levels of the acceptors, with ferrocene/ferrocenium
(Fc/Fc+) as the reference (Figure S5, ESI†). The HOMO/LUMO levels were
estimated using the following equations: HOMO = −e [Eox − E(Fc/Fc+) + 4.8] (eV)
and LUMO = −e [Ered − E(Fc/Fc+) + 4.8] (eV), where Eox and Ered are the onsets of
+
oxidation and reduction, respectively, and the potential of the FC/FC redox
couple was determined as around 0.3 V relative to a Ag/Ag+ reference electrode.
In this regard, the HOMO/LUMO levels of IDT-IC and IDT-HN were estimated to
be −5.84/−3.95 and −5.92/−3.86 eV, respectively. The LUMO level of IDT-HN is
higher than that of IDT-IC due to the electron-donating nature of the cyclic alkyl
group. IDT-IC and IDT-HN exhibited comparable LUMO offset of about 0.4 eV
with copolymer PBDB-T, which can guarantee the driving force for efficient
exciton dissociation.43 We also note that the recorded electrochemical band gap
cv
(Egcv) of IDT-HN (2.06 eV) is slightly higher than that of IDT-IC (Eg = 1.89 eV),
10

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while they have nearly identical optical band gap (Figure 1c). This observation
can be understood since the optical band gap demonstrates the exciton energy
which determines the onset of vertical interband transitions, while the
electrochemical band gap is the minimal energy needed to generate an electron
CV
hole pair. Another reason that may cause the different Eg between the two
compounds might be attributed to the contact between the film of IDT-HN and
the electrode, since the cyclic alkyl chains of the IDT-HN has large steric
hindrance that may increase the injection potentials of electrons during the
measurements, thus leading to the deeper HOMO and shallower LUMO energy
level of the IDT-HN that those of IDT-IC.
2.4. Photovoltaic Properties
With broadened and intensified absorption and elevated LUMO energy level, it is
reasonable to expect that IDT-HN would show a higher photovoltaic performance
than that of IDT-IC (Figure 1a). We fabricated OSCs with a conventional
configuration of ITO/PEDOT:PSS/active layer/PFN-Br/Ag, in which
poly[(9,9-bis(3-((N,N-dimethyl)-N-ethylammonium)propyl)-2,7-fluorene)-alt-2,7
-(9,9-dioctylfluorene)] (PFN-Br) functioned as an efficient cathode interlayer to
facilitate electron extraction from the active layer.44 The details of the device
optimization processes of the OSCs, including the donor/acceptor ratios,
active-layer thickness, and solvent additives, are provided in Tables S4 and Table
S5 (ESI†). The optimized performance was achieved when using chlorobenzene
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as the processing solvent with a donor:acceptor weight ratio of 1.5:1 and
active-layer thickness of around 100 nm with 0.5% DIO (1,8-diiodooctane) as the
solvent additive.
The optimized current density–voltage (J–V) curves and external quantum
efficiency (EQE) curves of the OSCs based on PBDB-T:IDT-IC and PBDB-T:IDT-HN
are shown in Figure 3, and the corresponding photovoltaic parameters are
summarized in Table 1. The optimized IDT-HN-based device exhibited an
impressively high PCE of 10.22% with a VOC of 0.93 V, a JSC of 14.43 mA cm−2, and
an FF of 76.41%. Compared with the IDT-IC-based device, the IDT-HN-based
device exhibited simultaneously enhanced VOC and JSC, which can be attributed to
the up-shifted LUMO level and wider and stronger absorption of IDT-HN. Note
that the device also showed a significantly increased FF of over 76%, which is
among the highest values of so far reported NF-OSC.35,45 In the EQE spectra, the
IDT-HN-based device shows a wider and stronger photoresponse than that of the
IDT-IC-based device, which can likewise be ascribed to the stronger absorption
of IDT-HN, and which is favorable for achieving high JSC. The EQE responses were
consistent with the measured JSC values of the devices. The integrated
photocurrents were 11.40 and 14.13 mA cm−2 for the PBDB-T:IDT-IC and
PBDB-T:IDT-HN devices, respectively, which are very close to the values of 11.94
and 14.43 mA cm−2 obtained from the J–V characterization, respectively (Table 1).
It is also worth pointing out that the EQE response of device based on IDT-HN
slightly red-shifted regarding to that of device based on IDT-IC when processing
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with DIO additive (Figure 3b), which can be understood as the UV-vis absorption
of the PBDB-T:IDT-HN blend film red-shifted regarding to that of PBDB-T:IDT-IC
blend film (Figure S4, SI).
Table 1. Photovoltaic properties of the PSCs based on PBDB-T as donor and IDT-IC, IDT-N or
IDT-HN as acceptor under AM 1.5 G at 100 mW cm-2
a
Active layer
VOC (V)
JSC (Jcalc
)
FF (%)
PCE (%)
Ref.
(mA cm-2)
PBDB-T:IDT-IC
PBDB-T:IDT-N
PBDB-T:IDT-HN
0.88
0.79
0.93
11.94 (11.40)
15.88 (15.17)
14.43 (14.13)
57.90
71.91
76.41
6.11
8.99
This work
33
10.22
This work
a The JSC in parentheses were calculated by integrating the EQE spectrum with the AM 1.5 G
spectrum.
5
80
70
60
50
40
30
20
10
0
(a)
PBDB-T:IDT-IC
PBDB-T:IDT-HN
(b)
0
-5
PBDB-T:IDT-IC
PBDB-T:IDT-HN
-10
-15
-0.2
0
0.2 0.4 0.6 0.8
Voltage(V)
1
300 400 500 600 700 800
Wavelength (nm)
Figure 3. (a) J–V curves and (b) EQE spectra of devices based on PBDB-T:IDT-IC, and
PBDB-T:IDT-HN.
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The dependence of device performances of IDT-HN-based OSCs on film
thickness (from 65 to 250 nm) was also measured to evaluate their potential for
practical applications. The devices were fabricated by carefully tuning the
spin-coating speed and the concentration of the blended solution (Table S5, ESI†).
As shown in Figure S7 (ESI†), PCE was dramatic increased from 8.52% to 10.22%
upon the film thickness increased from 65 to 100 nm, respectively, after which
the PCE values slowly decreased with film thickness. It is also worth noting that
a PCE of 9.29% was obtained at film thickness of 250 nm, indicating the great
potential for fabricating thick film devices that is compatible for high-throughput
roll-to-roll printing technique.
2.5. Charge Transfer, Recombination, and Extraction
To probe the charge transfer between the donor and acceptor materials, we
measured the photoluminescence (PL) spectra of the pure IDT-IC, IDT-HN, and
PBDB-T and optimized PBDB-T:IDT-IC and PBDB-T:IDT-HN blend films as shown
in Figure 4. Clearly, more effective photoinduced charge transfer occurred
between PBDB-T and IDT-HN compared with that between PBDB-T and IDT-IC,
which is beneficial for achieving a higher JSC in OSCs. This result is consistent
with the significantly enhanced JSC obtained from the J–V characterization for the
IDT-HN-based devices.
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1.2
1
1.2
(b)
IDT-IC
PBDB-T:IDT-IC
IDT-HN
(a)
PBDB-T
1
0.8
0.6
0.4
0.2
0
PBDB-T:IDT-IC
PBDB-T:IDT-HN
PBDB-T:IDT-HN
0.8
0.6
0.4
0.2
0
All @ 610 nm
All @ 610 nm
650 700 750 800 850
Wavelength (nm)
650 700 750 800 850
Wavelength (nm)
Figure 4. PL spectra of thin films: (a) PBDB-T, PBDB-T:IDT-IC, and PBDB-T:IDT-HN (1.5:1,
wt:wt, with 0.5% DIO); (b) IDT-IC, IDT-HN, PBDB-T:IDT-IC, and PBDB-T:IDT-HN (1.5:1,
wt:wt, with 0.5% DIO).
Transient photocurrent (TPC) measurements were performed to probe how
the introduction of cyclic alkyl chains into the end group affected the charge
extraction time during the operation of the OSC devices. The measurements were
carried out at 0 bias, and the results are shown in Figure 5a. The charge
extraction time was measured to be 0.28 and 0.19 µs for the PBDB-T:IDT-IC and
PBDB-T:IDT-HN devices, respectively. The shorter charge extraction time for the
IDT-HN-based device indicates more efficient charge extraction from the
photoactive layer, which is favorable for improving the current density and thus
overall photovoltaic performance.46
The light-intensity (Plight) dependence of JSC and VOC were measured to
elucidate the charge recombination mechanism of the studied devices, wherein
Plight was varied from 5 to 100 mW cm−2 (Figure 5). The power-law dependence
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of JSC on illumination intensity can be expressed as JSC ∝ (Plight)S,47 in which S, the
exponential factor, tends to unity when the bimolecular recombination of the
charge carriers is weak. 48 The extracted S values for the IDT-IC- and
IDT-HN-based devices are 0.98 and 0.99, respectively (Figure 5c), indicating that
less bimolecular recombination occurred in the IDT-HN-based device, which can
be expected to improve JSC and FF. 15 In addition, the relationships between VOC
and Plight are shown in Figure 5d. The slope of VOC versus the natural logarithm of
Plight gives the value of kBT/q. Here kB, T, and q are the Boltzmann constant,
temperature (K), and elementary charge, respectively.49 For the device based
on PBDB-T:IDT-HN, the obtained slope is 1.23 kBT/q, which is lower than the
value of 1.41 kBT/q obtained from the device based on PBDB-T:IDT-IC. Typically,
when an additional mechanism such as trap-assisted recombination is involved, a
stronger dependence of VOC on Plight, with a slope greater than kBT/q, can be
observed.47 Thus, the lower slope for the IDT-HN-based device demonstrates
the reduced trap density and suppressed trap-assisted recombination, which is
consistent with the obtained higher current density.
The exciton dissociation and charge-collection behavior of the studied
devices were investigated by measuring the photocurrent density (Jph) versus
effective voltage (Veff) as shown in Figure 5b. 50 Under the short-circuit
condition, the exciton dissociation capacity was evaluated by determining the
exciton dissociation probability (Pdiss = Jph/Jsat, where Jsat is the saturation
photocurrent density). Pdiss was 98% for the IDT-HN-based device, much higher
16

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than the value of 91% for the IDT-IC-based device, indicating that theDOfoI:r1m0.10e3r9/C8TC01780H
undergoes more efficient exciton dissociation and charge collection.
(a)
(b)
1
0.8
0.6
0.4
0.2
0
PBDB-T:IDT-IC
PBDB-T:IDT-HN
10
1
0.28 μs
0.19 μs
PBDB-T:IDT-IC
PBDB-T:IDT-HN
0.1
0.01
0.1
1
-1
0
1
2
3
4
5
V
(V)
Times (μs)
eff
0.95
0.9
100
10
1
(d)
(c)
PBDB-T:IDT-IC
PBDB-T:IDT-IC
PBDB-T:IDN-HN
PBDB-T:IDT-HN
α = 0.993
0.85
0.8
n = 1.229 kBT/q
α = 0.980
n = 1.407 kBT/q
0.75
0.1
10
Light intensity (mW cm )
100
10
Light intensity (mW cm )
100
-1
-1
Figure 5. For the optimized devices based on PBDB-T:IDT-IC or PBDB-T:IDT-HN: (a)
Transient photocurrent measurements; (b) Jph versus Veff plots; (c) JSC as a function of light
intensity; and (d) VOC as a function of light intensity.
2.6. Charge Carrier Mobility
The charge carrier mobilities of the pristine IDT-IC and IDT-HN films and the
PBDB-T:IDT-IC and PBDB-T:IDT-HN blend films were measured via the
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space-charge limited current (SCLC) method. The electron-only and hole-only
mobilities were evaluated by using devices with ITO/Al/BHJ film/Ca/Al and
ITO/PEDOT:PSS/BHJ
film/MoO3/Ag
architectures,
respectively.
The
current–voltage (J1/2–V) curves and the corresponding mobility parameters are
summarized in Figure 6, Figure S6, and Table S3, respectively. The electron
mobility of the IDT-HN film (2.79 × 10−4 cm2 V−1 s−1) is higher than the IDT-IC
film (1.35 × 10−4 cm2 V−1 s−1), which can be mainly attributed to the improved
intermolecular interactions and better π-π stacking, as inferred from GIWAXS
measurements (see below), thus ensuring effective charge carrier transport to
the electrodes. Relative to the PBDB-T:IDT-IC blend film (hole mobility, μh = 9.64
× 10−4 cm2 V−1 s−1; electron mobility, μe = 2.42 × 10−4 cm2 V−1 s−1; μh/μe = 3.98),
the PBDB-T:IDT-HN film possesses higher mobilities (μh = 1.04 × 10−3 cm2 V−1 s−1;
μe = 4.05 × 10−4 cm2 V−1 s−1) and also more balanced hole/electron mobilities
(μh/μe = 2.57), which are favorable for suppressing bimolecular recombination
and increasing the FF values.9
80
70
60
50
40
30
130
120
110
100
90
(a)
(b)
PBDB-T:IDT-IC
PBDB-T:IDT-HN
PBDB-T:IDT-IC
PBDB-T:IDT-HN
80
70
60
50
1.5
2
2.5
3
3.5
4
1
1.2 1.4 1.6 1.8
2
2.2 2.4
Vappl-Vbi-Vs (V)
V
-V -V (V)
appl bi
s
Figure 6. (a) Electron-only and (b) hole-only J1/2–V characteristics of PDBT-T:IDT-IC and
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DOI: 10.1039/C8TC01780H
PDBT-T:IDT-HN blend films.
2.7. Morphology Characterization
The internal physical mechanisms of blend films are known to be closely related
to their morphological features. Therefore, atomic force microscopy (AFM) and
grazing incidence wide-angle X-ray scattering (GIWAXS) were used to investigate
the effects of end-group modulation on the film morphology. As shown in Figure
S8 (ESI†), the IDT-HN-based blend film exhibited a mostly uniform and smooth
surface with a relatively low root-mean-square (RMS) roughness value of 2.72
nm. For the IDT-IC-based blend film, higher RMS roughness (RMS = 5.11 nm) and
larger surface aggregates were observed. Moreover, it is noted that these blend
films processed with and without DIO additive exhibited quite similar surface
topography associated with similar root-mean-square values, implying that the
utilization of solvent additive has trivial effects on the surface morphology of these
blend films.
Two-dimensional GIWAXS patterns of the neat films and the blends were
captured to investigate the molecular ordering characteristics (Figure 7). One can
clearly note that the neat film of IDT-IC exhibited a diffuse lamellar packing (100)
ring and a low-intensity π-π stacking (010) reflection peak at 1.77 Å−1 in the
in-plane (IP) direction, implying the formation of a predominant “edge-on”
orientation (Figure 7). In contrast, the film of IDT-HN showed a pronounced
lamellar diffraction (100) peak at 0.35 Å−1 with an interlamellar distance of 17.7
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Å in the IP direction and a high-intensity π-π stacking (010) peak at around 1.76
Å−1 with a d-spacing of 3.6 Å in the out-of-plane (OOP) direction, indicating the
formation of the predominant “face-on” orientation. For the blends, the
diffraction signals showed a (100) peak at 0.31 Å−1 and a π-π stacking (010) peak
at 1.7 Å−1, which can be attributed to the highly ordered polymer PBDB-T.15 It is
notes that the (010) peaks (q = 1.77 Å−1) in the out-of-plane direction and the (100)
peak (q = 0.3 Å−1) in the in-plane direction, slightly enhanced for the film processed
with DIO additive (Figure 7b, e) regarding to that of film processed without DIO
additive (Figure S9a, c). This observation indicated that the blend films processed
with DIO additive have more preferential to form the face-on orientation relative to
the substrate, which is consistent with the improved short-circuit current density of
devices. Moreover, compared with the IDT-IC-based film processed with 0.5%
DIO additive, the IDT-HN-based film (processed with 0.5% DIO) also showed a
stronger intensity of the (100) peak in the IP direction and of the π-π stacking
(010) reflection peak in the OOP direction, suggesting a stronger ordering of the
face-on orientation of both acceptor and donor. Such ordering is known to be
beneficial for the charge transport in OSCs,51,52 the results therefore correlating
well with the device characteristics. This finding confirm that the HN end group
can induce more favorable molecular orientation and film morphology, and
thereby resulting in better device performance.
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Figure 7. (a−d) 2D-GIWAXS patterns and (e) line-cut profiles of the in-plane (red line) and
out-of-plane (black line) direction of neat IDT-IC and IDT-HN, and PBDB-T:IDT-IC and
PBDB-T:IDT-HN blend films processed with 0.5% DIO additive.
3. Conclusion
In summary, a novel A-D-A-type small-molecule acceptor, denoted IDT-HN, with
IDT as the core and incorporating HN as a new electron-withdrawing end group,
was designed and synthesized. The resulting IDT-HN exhibited a broadened
and intensified optical absorption, enhanced electron mobility, elevated LUMO
energy level, and more ordered molecular packing than the INCN-end-capped
counterpart compound IDT-IC. Device based on PBDB-T:IDT-HN exhibited an
impressively high PCE of 10.22% that obviously outperforms that of 6.11%
obtained from the IDT-IC-based device, as can be attributed to the
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simultaneously enhanced photovoltaic parameters as a result of the improved
absorption, up-shifted LUMO energy level, the achievement of more ordered π-π
packing of molecules, the higher exciton dissociation efficiency, and more
balanced charge transport of the former. It is also worth noting that the PCE
remain over 9% at film thickness of 250 nm, illustrating a great potential for
future practical applications. These findings indicate that introducing cyclic
alkyl chains to the end groups of small-molecule acceptors is an effective way to
enhance the absorption, elevate the LUMO level, and optimize the film
morphology.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was financially supported by the Ministry of Science and Technology of
China (No. 2014CB643501), Natural Science Foundation of China (No. 91633301,
51673069, 21634004), the Science and Technology Program of Guangzhou, China
(No. 2017A030306011 and 2017A050503002), Foundation of Guangzhou Science
and Technology Project (201707020019). L.Y. thanks the Pearl River S&T Nova
Program of Guangzhou (Grant 201710010021).
Notes and references
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DOI: 10.1039/C8TC01780H
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Table of Contents Entry
0
PBDB-T:IDT-IC
PBDB-T:IDT-HN
-5
-10
-15
IDT-IC
PCE = 6.11%
C6H13
C6H13
O
CN
CN
S
S
CN
CN
PCE = 10.22%
O
C6H13
C6H13
0
0.2 0.4 0.6 0.8
Voltage(V)
1
IDT-HN